JPH09304144A - Drive circuit for flow rate sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a drive circuit which can be driven stably for a long period without using a bridge circuit which is apt to become unstable with the passage of time regarding a flow rate sensor which is constituted of one heating resistor and of one resistance temperature sensor. SOLUTION: A drive circuit is constituted in such a way that a resistor R1 , for measurement of a heating-body current, whose resistance value is known is connected in series with a heating resistor RH, that a current which is controlled via a CPU 19, a D/A converter 17 and a current source 18 is made to flow to a resistance temperature sensor RS, that the voltage of the resistance temperature sensor RS is used as a reference input, that the heating resistor RH and the resistor R1 for measurement of the heating-body current are driven by a voltage source 21, that a voltage across both ends of the resistor R1 for measurement of the heating-body current is measured via an A/D converter 23 by changing over an analog switch 24, that a current value which flows in the heating resistor RH is acquired, that the voltage of the heating resistor RH is measured and that the resistance value of the heating resistor RH can be measure even when the heating resistor RH is heated and driven.

Description

Detailed Description of the Invention

[0001]

[0001] The present invention relates to city gas and LPG.
It can be applied to the flow rate measurement and the measurement of the amount of air taken into an internal combustion engine of an automobile, and both have a large temperature coefficient of resistance and one heat-generating resistor and one temperature-measuring resistor arranged in the flow path. The present invention relates to a flow rate sensor drive circuit in a flow rate measuring device using a flow rate sensor including the.

[0002]

2. Description of the Related Art Conventionally, there are various types of flow rate sensor driving circuits in a flow rate measuring apparatus of this type. One of them is a Wheatstone bridge circuit using a heating resistor and a temperature measuring resistor. There is a method to measure the flow rate by the voltage of the heating resistor. This method is disclosed, for example, in Japanese Examined Patent Publication No. 5-313616.

[0003] In addition, for example, Japanese Patent Publication No. 7-1
As shown in Japanese Patent No. 8727, a heating resistor and a temperature measuring resistor (reference resistor) are used, but there is also a system in which a circuit is configured to measure a flow rate without configuring a bridge circuit.

A typical example of the flow sensor drive circuit using the former bridge circuit will be described with reference to FIG. The circuit shown in FIG. 9 is different from the circuit shown in Japanese Patent Publication No. 5-316616, but the basic principle is the same. First, a heating resistor R _h is provided on a bridge that extends over a moat formed on a flow sensor chip substrate along a flow path direction, and a heating resistor R _h is arranged on the flow sensor chip upstream of the heating resistor R _h. The resistance temperature detector R _s provided is provided. Both the heating resistor R _h and the temperature measuring resistor R _s have a high resistance temperature coefficient, and form the Wheatstone bridge circuit 1 together with the reference resistors R ₁₁ and R ₁₂ whose resistance values are fixed. .
Here, the temperature measuring resistor R _s and the reference resistor R ₁₁ are connected in series, and the heating resistor R _h and the reference resistor R ₁₂ are connected in series. A reference resistor R ₁₃ for determining the heating temperature of the heating resistor R _h is connected between the reference resistor R ₁₁ and the temperature measuring resistor R _s . A switch 2 controlled to be short-circuited or opened is connected in parallel to the reference resistor R ₁₃ . The reference resistor R _11, R these reference resistor is between ₁₂ R _11, R ₁₂ together with the resistor R _h, variable resistor VR for balance fine adjustment of the current flowing through the R _s are connected.

Furthermore, the connection point a of the reference resistors R ₁₁ and R ₁₃
There is provided a differential amplifier 3 for comparing the voltage at the point b and the voltage at the connection point b between the reference resistor R ₁₂ and the heating resistor R _h .
The differential amplifier 3 outputs a zero-balanced output from the terminal 4, and the output of the differential amplifier 3 is connected to the power control element, for example, the base side of the power control transistor 5. R ₁₄ and R ₁₅ are bias resistors. Here, the collector and the emitter of the power control transistor 5 are connected between the DC power source 6 having the voltage Vcc and the variable slider of the variable resistor VR. The power control transistor 5
A switch 7 is interposed between the emitter of the variable resistor and the variable slider of the variable resistor VR. A high resistance resistor R ₁₆ is connected between the other end of the switch 7 and the DC power source 6 (that is, in parallel between the collector and the emitter of the power control transistor 5).

There is also provided a differential amplifier 8 which receives each voltage across the heating resistor R _h as an input.

In such a configuration, in the circuit adjustment mode, the switch 7 is switched to the left side in the drawing to select the resistor R ₁₆ , and the switch 2 is closed to short-circuit both ends of the reference resistor R _13. And As a result, the heating resistor R is passed from the DC power source 6 through the resistor R ₁₆ having a high resistance value.
_A small current is passed through _h and the resistance temperature detector R _s , and the differential amplifier 3
The variable resistor VR and the like are adjusted so that the output obtained at the terminal 4 of is zero-balanced.

Then, in the actual flow rate measuring mode, the switches 2 and 7 are switched to the illustrated state. That is, the reference resistor R ₁₃ is released from the short circuit, and the power control transistor 5 is switched to operate. At this time, the voltage at the connection point a is increased by adding the voltage component of the reference resistor R ₁₃ to the resistance temperature detector R _s, and the zero balance of the bridge circuit 1 is lost, so that the amount of electricity supplied to the heating resistor R _h is increased. The power control transistor 5 is controlled via the differential amplifier 3 so that the voltage at the midpoints a and b is zero-balanced again due to the increase in the resistance value and the increase in the resistance value due to the heat generation. That is, the reference resistor R ₁₃
Is connected, the negative feedback control is performed so that the temperature of the heating resistor R _h is higher than the temperature of the resistance temperature detector R _s by a constant temperature (constant temperature heating drive).

[0009] Measurement of flow rate is performed by detecting a voltage drop across the heating resistor R _h in such flow measurement mode through the differential amplifier 8. That is, as the flow rate is higher (therefore, the flow rate is faster), the heating resistor R _h is cooled by the fluid, so that the heating temperature is maintained in order to maintain the constant heating temperature defined by the reference resistor R ₁₃ . By increasing the power consumption by the resistor R _h , the heating resistor R
_{Since the} negative feedback control is performed so that the voltage drop due to _h is proportional, the output voltage obtained at the terminal 9 of the differential amplifier 8 also increases. Therefore, if the slope of the output voltage is known in advance, the flow rate (flow velocity) can be measured.

[0010]

However, in the case where the bridge circuit 1 as shown in FIG. 9 is assembled, the power control transistor 5 is connected to the inverting terminal of the differential amplifier 3.
There is no particular problem because negative feedback is applied via the feedback amplifier, etc., but on the non-inverting terminal side of the differential amplifier 3, a slight fluctuation due to the temperature rise of the resistance temperature detector R _s is fed back as positive feedback, so feedback is performed. There is a drawback that the output voltage tends to become unstable depending on the time constant of the system.

In general, the reference resistor R ₁₁ ,
Even if the resistance values of R ₁₂ , R _13, etc. are stable, the heating resistor R
The resistance values of _h , the resistance temperature detector R _{s, and the} like change over time, which affects the measurement. In other words, as shown in FIG. 9, the heating resistor R _h and the temperature measuring resistor R _{h
When the} reference resistors R ₁₁ , R ₁₂ , and R ₁₃ are used for the s to form a bridge structure, the heat resistance R _h and the resistance temperature detector R _s are adjusted with time by digital signal analysis. It is not possible to take flexible measures such as continuing to measure.

By the way, according to the latter Japanese Patent Publication No. 7-18727, the reference voltage for the differential amplifier is obtained by dividing the power supply voltage with a series circuit of a fixed resistor and a reference resistor (corresponding to a temperature measuring resistor). A voltage is generated and the voltage of the heating resistor is fed back to the differential amplifier. However, in this configuration, when constant temperature heating drive is desired, it is not possible to know the resistance value of the heating resistor that has changed during heating only by the voltage of the heating resistor, so that the resistance value of the heating resistor can be measured. However, there is no mention of how to deal with it. Furthermore, a fixed resistor connected in series with the reference resistor is used to generate the reference voltage, but in order to adjust the circuit,
If necessary, you may want to change the current that flows in the resistance temperature detector (reference resistor), or to change the voltage input to the differential amplifier even if this current is constant. I can't get it. Further, as in the former case, it is not possible to flexibly deal with changes with time of the heating resistor and the temperature measuring resistor.

[0013]

According to the first aspect of the present invention,
In a flow rate sensor drive circuit that uses a flow rate sensor that has a large resistance temperature coefficient and one heating resistance element and one temperature measuring resistance element that are arranged in the flow path, the flow rate sensor driving circuit is connected in series with the heating resistance element. Resistor with a fixed resistance value and a known value, a reference voltage generator for generating a reference voltage, and D to which the output voltage from this reference voltage generator is input
/ A converter, a current source for energizing and driving the resistance temperature detector with a current according to the output voltage of the D / A converter, and the heating element current using the voltage obtained at the resistance temperature detector as a reference input. A voltage source for energizing and driving a series circuit of the measuring resistor and the heating resistor, and measuring the voltage across the heating resistor current measuring resistor and converting it to a digital voltage value, and the voltage of the heating resistor. Measure and convert to a digital voltage value A
/ D converter, an analog switch for switching between the measurement of the voltage across the heating element current measuring resistor and the measurement of the voltage of the heating resistor, and the digital voltage value output from the A / D converter And a central processing unit for controlling the switching of the analog switch and the operation of the D / A converter.

Therefore, the D / A controlled by the central processing unit
The converter drives the current source with the reference voltage generated by the reference voltage generator as an input, so that the current source energizes and drives the resistance temperature detector. The voltage source drives the heating resistor based on the voltage generated in the resistance temperature detector. At this time,
Since a heating element current measuring resistor whose resistance value is known is connected in series with the heating resistor, the heating resistor is measured by measuring the voltage across the heating element current measuring resistor through an A / D converter. You can get the current value flowing through your body. In addition, the voltage of the heating resistor is changed to A / D by switching the analog switch.
By measuring through the converter, using the obtained current value, the actual resistance value of the heating resistor during driving, that is,
The exothermic temperature can be known. Therefore, even if there is a change with time in the heating resistor, the output of the current source driving the resistance temperature detector in real time is adjusted by the D / A converter under digital control by the central processing unit. This makes it possible to adopt flexible control such as applying comprehensive correction using digital control in response to changes in the heating resistor due to long-term use. The actual flow rate can be obtained based on the voltage-flow rate characteristic previously acquired by the central processing unit by using the digital voltage value obtained based on the voltage measurement of the heating resistor.

According to a second aspect of the present invention, the voltage source according to the first aspect has a feedback connection for returning the potential at the connection point between the heating element current measuring resistor and the heating resistor. Therefore, even if the heating element current measuring resistor is connected in series with the heating resistor, the voltage flowing to the voltage source input is measured while measuring the current flowing through the heating resistor without affecting the driving voltage of the heating resistor. Therefore, the driving voltage of the heating resistor can be stably generated.

According to a third aspect of the invention, the gain of the current source or the reference voltage generator according to the first aspect is controllable by a central processing unit, and the A / D converter is located downstream of the analog switch. Immediately before this, there is an amplifier whose gain is controllable by the central processing unit. Therefore, the driving voltage of the heating resistor is greatly different between the measurement mode and the adjustment mode, but by switching the gain in both modes,
It is possible to deal with the low voltage output in the adjustment mode without lowering the resolution of the A / D converter or the D / A converter.

According to a fourth aspect of the present invention, the analog switch according to the first aspect is connected in parallel to the heating element current measuring resistor, and in the measurement mode, both ends of the heating element current measuring resistor are short-circuited for adjustment. Switching is controlled by the central processing unit so as to be opened in the mode, and the input terminal of the A / D converter is connected to the connection point between the voltage source and the heating element current measuring resistor. Therefore, when the resistance value of the heating element is not measured, the voltage drop due to the heating element current measuring resistor can be eliminated by closing the analog switch and short-circuiting both ends of the heating element current measuring resistor. Therefore, the dynamic range for driving the heating resistor can be widened even at a low voltage.

[0018]

FIG. 1 shows a first embodiment of the present invention.
A description will be given with reference to FIG. First, as shown in FIG. 2, the flow rate sensor 11 used in the present embodiment is integrally mounted on the end portion of the sensor substrate 12, and is disposed in the flow path 13 that is the flow rate measurement target. Here, basically, when the cross-sectional area of the flow path 13 is S and the velocity of the fluid is V, the flow rate Q to be measured by the flow rate sensor 11 is Q = V × S.

An example of a planar structure of the flow sensor 11 is shown in FIG. In the flow rate sensor 11, a moat 14 is formed along the direction of fluid flow, and a bridge 15 is formed so as to straddle the moat 14. A heating resistor R _H is patterned on the bridge 15 with a material having a high temperature coefficient of resistance. That is, the heating resistor R _H is formed so as to be sufficiently exposed to the flow of the fluid flowing inside the moat 14 and on the bridge 15. Further, in the flow rate sensor 11, the resistance temperature detector R _S is formed in a pattern by being positioned on the most upstream side in the direction of fluid flow and made of a material having a high resistance temperature coefficient.

Next, an example of the structure of a drive circuit for the flow rate sensor 11 including the heating resistor R _H and the temperature measuring resistor R _S will be described with reference to FIG. First, the reference voltage element 16 is provided as a reference voltage generator that generates a predetermined reference voltage. A D / A converter 17 and a current source 18 are sequentially connected to the reference voltage element 16. D / A
The converter 17 is connected to a CPU 19 which is a central processing unit via a data line 20 to control the D / A conversion amount. The current source 18 energizes and drives the resistance temperature detector R _S with a current according to the value of the output voltage output from the D / A converter 17, and the output side of the current source 18 has the One end of the resistance temperature detector R _S is connected. Further, there is provided a voltage source 21 to which a voltage obtained at one end of the resistance temperature detector R _S is inputted as a reference voltage which is a reference input. This voltage source 21 is a differential amplifier 2
2 and the like, and the heating resistor R _H is connected to the output side thereof, but between the heating resistor R _H and the voltage source 21, the resistor R for measuring the heating element current is connected. ₁ is connected. That is, the heating element current measuring resistor R ₁ is connected in series to the heating resistor R _H , and its resistance value is fixed and has a known value. As will be described in detail later, the potential at the connection point between the heating element current measuring resistor R ₁ and the heating resistor R _H is negatively feedback connected to the inverting input terminal of the differential amplifier 22.

Further, an A / D converter 23 for measuring a voltage of the heating resistor R _H and the heating current measuring resistor R ₁ and converting the voltage into a digital voltage value is supplied via an analog switch 24. It is provided. The analog switch 24 includes first terminals T ₁ and T ₂ drawn from both ends of the heating element current measuring resistor R ₁ and the heating resistor R _H.
And the second terminals T ₃ and T ₄ drawn out from both ends of the first terminal T 2 are connected to the A / D converter 23 under the control of the CPU 19 through the control line 25.
It is switched between ₁ , T ₂ and the second terminals T ₃ , T ₄ . Here, the terminal T ₃ is a terminal having the same potential as the terminal T ₂ , and the terminal T ₄ is grounded. More specifically, as shown in FIG. 4, a sample and hold circuit (S / S) whose sample and hold operation is controlled by the CPU 19 via the control line 25 on the lead lines for the terminals T ₁ and T ₃ .
H) 26 and 27 are interposed. A suitable low-pass filter (LP
F) is provided.

The CPU 19 connected to the D / A converter 17 and the A / D converter 23 includes the CPU 19
ROM or RAM that stores in advance program data for restricting the operation of the
The memory 28 configured by is connected.

In such a configuration, the D / A converter 17 whose output level is controlled by the CPU 119 inputs the output of the reference voltage element 16 as a reference voltage input and operates the current source 18. Thus, current source 18 RTD R _S drives the energization, the resistance temperature detector R voltage source 21 heating resistor a voltage produced at one end as a reference voltage _S R
Energize _H. Here, since the heating resistor R _H heating element current measuring resistor R ₁ are connected in series, the heating element current current of the heating resistor R _H and the same amount in the measuring resistor R ₁ is Flowing. Therefore, the analog switch 24 is switched to the side of the first terminals T ₁ and T ₂ , the voltage across the resistor R ₁ for measuring the heating element current is measured by the A / D converter 23, and the corresponding digital voltage value is obtained. Since the resistance value of the heating element current measuring resistor R ₁ is known, the value of the current flowing through the heating element current measuring resistor R ₁ and therefore the temperature measuring resistor R _S
The current value flowing through the CPU 19 can be known.
Also, the analog switch 24 is switched to the second terminals T ₃ and T ₄ side, and the A / D converter 23 causes the heating resistor R _H.
When the voltage is measured and the corresponding digital voltage value is obtained, the CPU 19 can obtain the actual resistance value of the heating resistor R _H by the calculation with the previously obtained current value.

In such a measuring operation, since the sample and hold circuits 26 and 27 including a low pass filter are provided in the preceding stage of the measuring system for the A / D converter 23, the heating element current measuring resistor R is provided. _{It is} possible to ensure the simultaneity when measuring the voltage across _{1 and} the voltage of the heating resistor R _H , and to stabilize the sampling operation of the A / D converter 23. Further, the potential at the connection point between the heating element current measuring resistor R ₁ and the heating resistor R _H (hence, the heating resistor R _H
H voltage) is negatively feedback-connected to the input side of the differential amplifier 22, so that the voltage according to the reference input to the voltage source 21 is not affected by the voltage drop due to the heating element current measuring resistor R _1. Can be generated in the heating resistor R _H. That is, even in the presence of the heating element current measuring resistor R _1, without affecting the driving voltage of the heating resistor R _H, while measuring the current flowing through the heating resistor R _H, the differential amplifier 22 The drive voltage of the heating resistor R _H can be stably controlled according to the reference input of.

[0025] Therefore, CPU 19 retrieves the digital information obtained from the A / D converter 23, and calculates the temperature of the heating resistor R _H, and stores the result in memory 28, the heating resistor R _H The digital voltage value based on the measured voltage value is used to calculate the fluid flow rate based on the previously known voltage-fluid flow rate relationship.

In this way, according to the present embodiment,
During fever driving of the heating resistor R _H, it is possible to measure the resistance value of the heating resistor R _H, if the resistance value of the heating resistor R _H as there is temporal change, real-time This can be dealt with by easily adjusting the output of the current source 18 driving the resistance temperature detector R _S through the D / A converter 17. That is, in response to changes in the heating resistor R _H due to long-term use, even during use, comprehensive correction using digital information under the control of the CPU 19 can be performed, and flow rate measurement can be performed over a long period of time. It can be performed stably across.

By the way, the voltage source 2 in the present embodiment
FIG. 5 shows an example of the detailed configuration around 1. A bipolar transistor 29 is connected to the output side of the differential amplifier 22 via bias resistors R ₂ and R ₃ for base and collector / base. The collector side of the transistor 29 is connected to the power supply Vcc, and the heating element current measuring resistor R ₁ and the heating resistor R _H are connected to the emitter side.
With this configuration, the heating resistor R _H current corresponding to the output voltage applied to the base of transistor 29 is passed by the differential amplifier 22, the voltage of the heating resistor R _H (heating element current measuring resistor Negative feedback control is performed so that the potential of the connection point between R ₁ and the heating resistor R _H ) matches the reference input (voltage of the temperature measuring resistor R _S ) input to the differential amplifier 22.

Note that, instead of the bipolar type transistor 29 as shown in FIG. 5A, a transistor 30 of FET is used as shown in FIG.
You may make it drive _H. As shown in FIG. 5B, the use of the transistor 30 of FET makes it possible to use two resistors (heater current measuring resistor R ₁ and heat generating resistor) even if the voltage of the power source Vcc is low or constant. The dynamic range of driving the body R _H ) can be widened.

Next, a second embodiment of the present invention will be described with reference to FIG. The same parts as those shown in FIGS. 1 to 5 are denoted by the same reference numerals, and the description thereof will be omitted (the same applies to each of the following embodiments). In the present embodiment, instead of the current source 18, a current source 31 whose gain is variable is provided.
Is used. Further, on the output side of the analog switch 24 and immediately before the input side of the A / D converter 23,
Amplifiers 32 and 33 having variable gains are provided. The gains of the current source 31 and the amplifiers 32 and 33 are controlled by the CPU 19 through the control line 25. Specifically, the CPU 19 lowers the gain of the current source 31 in the adjustment mode in which a low current flows,
In synchronization with this, control is performed so that the gains on the amplifier 32, 33 side are increased.

According to such a configuration, to generate heat actually the adjusting mode supplying a low current to the heating resistor R _H The heating resistor R _H in order to measure the resistance value of the heating resistor R _H Therefore, there is a big difference in the voltage of the heating resistor R _H between the measurement mode in which a relatively large amount of current flows, but the gain of the current source 31 is reduced in the adjustment mode, and in synchronization with this, the amplifier 32, By controlling the gain on the 33 side to be high, the D / A converter 17 and the A / D converter 23 can be operated without lowering the resolution. Therefore, it is possible to follow a slight change in the heating resistor R _H.

A third embodiment of the present invention will be described with reference to FIG. In the present embodiment, the reference voltage element 34 whose gain is variable is used in place of the reference voltage element 16. Further, similarly to the above-described embodiment, amplifiers 32 and 33 having variable gains are provided on the output side of the analog switch 24 and immediately before the input side of the A / D converter 23. These reference voltage element 34 and amplifier 3
The gains of 2 and 33 are configured to be controlled by the CPU 19 through the control line 25. Specifically, by controlling the value of the voltage input from the reference voltage element 34 to the D / A converter 17 by the CPU 19, the resolution of the D / A converter 17 does not decrease even in the adjustment mode. It will be.

A fourth embodiment of the present invention will be described with reference to FIG. In the present embodiment, the analog switch 24
Instead of this, an analog switch 35 is used which can short-circuit and open between both ends of the heating element current measuring resistor R ₁ .
The switching control of the analog switch 35 is also performed by the CPU 19.
It is performed by The input terminal of the A / D converter 23 is connected to the output side of the differential amplifier 22 and the heating element current measuring resistor R ₁
Is connected to the connection point. An amplifier 32 is connected on this connection line. The other input terminal of the A / D converter 23 is grounded.

In such a configuration, the CPU 19 opens the analog switch 35 to open the A / D converter 23.
When the voltage is measured by the resistor R ₁
The voltage applied to the heating resistor R _H is measured, and the voltage is measured by the A / D converter 23 in a state where the analog switch 35 is closed and the heating current measuring resistor R ₁ is short-circuited. The voltage across the body R _H is measured. Therefore, the voltage applied to the heating element current measuring resistor R ₁ can be measured by taking the difference between the two measured voltages. This allows
The value of the current flowing through the heating resistor R _H can be known.

On the other hand, in the flow rate measurement mode, the CPU 19 closes the analog switch 35 to short-circuit the resistor R ₁ for measuring the heating element current, and the measurement is performed while driving the heating resistor R _H. That is, since the analog switch 35 is closed, the CPU 19 acquires the voltage information of the heating resistor R _H via the A / D converter 23. Such upon driving of the heating resistor R _H, the heating resistor be R _H heating element current measuring resistor R ₁ connected in series to the be present voltage due to the heating element current measuring resistor R ₁ Since there is no drop, a large dynamic range can be secured when driving the heating resistor R _H. Further, since the voltage drop due to the heating element current measuring resistor R ₁ is not involved, it is possible to shorten the rising time of heating of the heating element R _H.

[0035]

According to the first aspect of the present invention, the voltage of the heating resistor is measured, and the voltage across the resistor of the heating element current measuring resistor having a known resistance value connected in series to the heating resistor is measured. Since the current value flowing through the heating resistor can be known by measuring, it becomes possible to measure the resistance value of this heating resistor even when the heating resistor is driven, and the resistance value of the heating resistor changes. When there is, the output of the current source driving the resistance temperature detector can be adjusted in real time by the D / A converter. Therefore, a digital signal is output in response to the change of the heating resistor due to long-term use. It is possible to flexibly respond by applying the comprehensive correction used, and it is possible to maintain stability for a long period of time.

According to the second aspect of the present invention, since the feedback connection for returning the potential at the connection point between the heating element current measuring resistor and the heating resistor is provided, the heating element current measurement is performed on the heating resistor. Even if the resistors are connected in series, the driving voltage of the heating resistor can be stabilized according to the voltage source input while measuring the current flowing through the heating resistor without affecting the driving voltage of the heating resistor. Can be generated.

According to the third aspect of the invention, the gain of the current source or the reference voltage element can be controlled by the central processing unit, and the gain is controllable on the downstream side of the analog switch in the A / D.
Since an amplifier whose gain can be controlled by the central processing unit is provided immediately before the converter, the driving voltage of the heating resistor is greatly different between the measurement mode and the adjustment mode, but the gain is appropriately adjusted in both modes. By switching, it is possible to cope with the low voltage output in the adjustment mode without lowering the resolution of the A / D converter or the D / A converter.

According to the fourth aspect of the present invention, the analog switch is connected in parallel to the heating element current measuring resistor so that both ends of the heating element current measuring resistor are short-circuited in the measurement mode and opened in the adjustment mode. As described above, switching control is performed by the central processing unit, and since the input terminal of the A / D converter is connected to the connection point between the voltage source and the heating element current measuring resistor, the resistance value of the heating resistor is When not measuring, the voltage drop due to the resistor for heating element current measurement can be eliminated by closing the analog switch and short-circuiting both ends of the resistor for heating element current measurement. The dynamic range for driving can be widened.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

FIG. 2 is a perspective view conceptually showing the overall structure of a flow rate measuring device.

FIG. 3 is a plan view conceptually showing the structure of a flow rate sensor portion.

FIG. 4 is a circuit diagram showing an input portion of an A / D converter extracted.

FIG. 5 is a circuit diagram showing the vicinity of a voltage source by extraction.

FIG. 6 is a circuit diagram showing a second embodiment of the present invention.

FIG. 7 is a circuit diagram showing a third embodiment of the present invention.

FIG. 8 is a circuit diagram showing a fourth embodiment of the present invention.

FIG. 9 is a circuit diagram showing a conventional example.

[Explanation of symbols]

 11 Flow Rate Sensor 13 Flow Path 16 Reference Voltage Generator 17 D / A Converter 18 Current Source 19 Central Processing Unit 21 Voltage Source 23 A / D Converter 24 Analog Switch 31 Current Source 32, 33 Amplifier 34 Reference Voltage Element

Claims (4)

[Claims] 1. A flow rate sensor drive circuit using a flow rate sensor having one heating resistor and one temperature measuring resistor, both of which have a large temperature coefficient of resistance and are disposed in a flow path, wherein the heating resistor is used. A resistor for heating element current measurement that is connected in series with a fixed resistance and has a known value, a reference voltage generator that generates a reference voltage, and a D / A to which the output voltage from this reference voltage generator is input.
A converter, a current source for energizing and driving the resistance temperature detector with a current according to the output voltage of the D / A converter, and the heating element current measurement using the voltage obtained in the resistance temperature detector as a reference input. A voltage source for energizing and driving a series circuit of a resistor and the heating resistor, and measuring the voltage across the resistor for measuring the heating element current and converting it into a digital voltage value, and measuring the voltage of the heating resistor. A / D converter for converting into a digital voltage value, an analog switch for switching between measurement of the voltage across the heating element current measuring resistor and measurement of the voltage of the heating resistor, and the A / D converter A flow rate sensor drive circuit comprising: a central processing unit that takes in the digital voltage value output from the control unit and controls the switching of the analog switch and the operation of the D / A converter. 2. The flow sensor drive circuit according to claim 1, wherein the voltage source has a feedback connection for returning the potential at the connection point between the heating element current measuring resistor and the heating resistor. 3. The gain of the current source or the reference voltage generator is controllable by a central processing unit, and the gain is controlled by the central processing unit downstream of the analog switch and immediately before the A / D converter. The flow sensor drive circuit according to claim 1, further comprising a flexible amplifier. 4. The central processing unit connects the analog switch in parallel with the heating element current measuring resistor so as to short-circuit both ends of the heating element current measuring resistor in the measurement mode and open in the adjustment mode. Switching control, A /
The flow sensor drive circuit according to claim 1, wherein an input terminal of the D converter is connected to a connection point between a voltage source and the heating element current measuring resistor.